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\documentclass[12pt]{article}
\usepackage{natbib}
\bibliographystyle{plainnat}
\bibpunct{[}{]}{,}{a}{,}{;}
\begin{document}

\textbf{Multiple References}

\begin{enumerate}
\item
\verb|\citet[see][]{goossens:companion,goossens:companion2,hagen:metafun}| ---
\citet[see][]{goossens:companion,goossens:companion2,hagen:metafun}
\item
\verb|\citep{goossens:companion,goossens:companion2,hagen:metafun}| ---
\citep{goossens:companion,goossens:companion2,hagen:metafun}
\item
\verb|\citealt[see][]{goossens:companion,goossens:companion2,hagen:metafun}| ---
\citealt[see][]{goossens:companion,goossens:companion2,hagen:metafun}
\item
\verb|\citealp[see][]{goossens:companion,goossens:companion2,hagen:metafun}| ---
\citealp[see][]{goossens:companion,goossens:companion2,hagen:metafun}
\end{enumerate}

\bibliography{natbib}

\end{document}
